Current technology solutions include 802.1Q (VLAN), IEEE 802.1ah (Provider Back-Bone Bridging) and 802.1D bridging.
These base solutions are notorious for making inefficient use of available links—because of the use of one (or more) variations of STP (Spanning Tree Protocol), which effectively blocks ports (and corresponding links)—making them unusable.
Proposals discussed to date to provide a means of making better use of redundant links include:                MSTP: MSTP makes use of VLAN grouping to derive distinct spanning trees—where one or more VLANs are associated with each spanning tree, ports are “blocked” on a VLAN group basis, and the choice of links that make up a VLAN group spanning tree are pseudo-randomized in an attempt to equalize the distribution of VLAN traffic across links. One problem with this approach is that MSTP does not currently define a means to weight the assignment of VLANs to a group, or the overlap of VLAN groups and all redundant-link choices, on the basis of either expected or observed traffic on each VLAN—consequently, the service quality received by network users is typically influenced by incidental factors used in random selection rather than the service quality they appear to expect.        802.1ah: 802.1ah is an IEEE proposal, for use by service providers, using MAC-in-MAC tunneling for the delivery of traffic on a provider-edge to provider-edge basis. Limitations of this approach include the fact that 802.1ah does not define or describe mechanisms for how the MAC-in-MAC tunnels are defined and configured and—consequently—how distribution of traffic is established and maintained.        PBT: PBT (Provider Backbone Transport) is a Nortel proposal that extends 802.1ah by providing 1) a means and basis for establishing 802.1ah tunnels and 2) methods for establishing and maintaining tunnels to accommodate specific service quality needs. Issues with this approach include the fact that control and forwarding across infrastructure bridges requires all such bridges to support the mechanism.        GELS: GELS (GMPLS [Generalized Multi-Protocol Label Switching] controlled Ethernet Label Switching) is a proposal (brought to the IETF from several sources). It suggests using a GMPLS control-plane to establish the equivalent of the combination of multiple spanning trees and local 802.1Q-like filtering databases. The essential mechanisms discussed to date would use signaling to program forwarding entries at each GELS-compliant GMPLS-controlled Ethernet switch, where both the VLAN tag and the destination MAC are used as keys (in signaling and in forwarding) for creating/finding each forwarding entry. The most important issue with GELS is that it is largely undefined at this point—including, in particular, how the collection of paths is determined prior to signaling (whether by central or distributed computation) and how such a determination impacts provisioning of service quality. A secondary issue is that—at present—it appears that all Ethernet forwarding devices within an Ethernet forwarding domain must be GELS-compliant.        TRILL: TRILL (Transparent Interconnect over Lots of Links) is an IETF proposal that combines 802.1D—as well as 802.1Q—and either IS-IS, or OSPF, routing to determine optimal forwarding paths (independent of any variation of STP) using SPF (Shortest Path First) routing mechanisms. The key objective of this effort is to define a scalable—SPF based—Ethernet forwarding approach for use in large enterprises. Current proposals in this effort focus on minimal configuration, use of Ethernet+SHIM in Ethernet encapsulation, and forwarding optimization based on link utilization, VLAN and (largely IP[Internet Protocol]-specific) Multicast Group forwarding. As this effort is currently targeted for Enterprise use, proposals do not include (but also do not preclude) use of a signaling or negotiation process to determine path selection on a basis other than SPF routing.        
As stated above, MSTP can easily result in (possibly pathologically) inequitable distribution of Ethernet traffic.
802.1ah addresses a broader problem space, allowing—but not proposing—a specific solution to the problem of traffic distribution and service quality.
Both PBT and GELS claim to provide a potential solution to the bandwidth distribution problem—through the use of signaling—but require ubiquitous deployment of the specific solution chosen.
TRILL offers a solution that provides path usage optimization and may be used in an incremental replacement/deployment strategy for large enterprises—but does not directly allow for modification of assignment of traffic to links on the basis of service quality expectations.
As part of the background, 802.1ah is derived from (and includes) 802.1Q—as 802.1Q is derived from (and includes) 802.1D. PBT is an extension, or proposed extension, to 802.1ah. GELS is a related—but different—proposal with respect to 802.1Q and may or may not include 802.1ah as it continues to develop in the IETF. TRILL is intended to extend 802.1Q, in combination with IS-IS.